Well there you go. You didn't follow correct procedure and had to wait longer for the outcome. Had you reported the buyer for breaching the abusive buyer policy, it would have taken far less than 3 weeks to have it sorted out. The buyer would have also had a breach recorded against their account - which they probably didn't get because you failed to follow the correct procedure.

At no stage did I say they "can't" open the dispute, what I said was they are not permitted to do so. When you encounter somebody blatantly breaching policy, use the tools provided to report them. It really is that simple.